max slomoff said:
and, if you have 2 150w bulbs shining on the same subject is the
brightness exactly equal to that of 1 300w bulb?
No, generally a 300W of the same life expectancy and filament style
produces slightly more than twice the light of a 150W.
Look at the lumen ratings of various wattages.
Lower wattages are less efficient for three reasons:
1. In higher wattage bulbs, a greater percentage of lighting cost is
electricity cost and a lower percentage is bulb replacement cost. Because
of this, it pays to run the higher wattage filament filaments hotter for
greater efficiency despite lower life expectancy.
2. In higher wattage bulbs the filament is thicker, and at a given
temperature within a given time a smaller percentage of the thicker
filament evaporates. The thicker filament can be run hotter for the same
life expectancy.
3. In gas-filled bulbs, the heat conduction from the filament by the gas
is surprisingly proportional to the visibly apparent filament length and
less than proportional to the visibly apparent overall filament diameter.
A wider filament has a thicker "boundary layer" of hot gas around it and a
correspondingly lower temperature gradient. With a thicker/wider
filament, a smaller percentage of the energy going into the bulb becomes
heat conducted from the filament by the gas.
In fact, below a certain "wattage per centimeter" of overall filament
length of maybe roughly 8-10 w/cm or so, they use a vacuum rather than a
gas because the gas hurts more than it helps. (When the gas is used, it
slows down filament evaporation by having gas atoms "bounce" evaporated
tungsten atoms back onto the filament to permit a higher filament
temperature for a given evaporation rate.)
- Don Klipstein (
[email protected])